It is well known that more engine parts are made of an aluminum alloy to satisfy the requirements such as reduction in weight or improvement in heat dissipation. Parts that slide in a reciprocating way at high speed, such as a piston head and a piston ring, exert inertia force that is proportional to the mass of themselves that has significant impact on their operating characteristics. Thus, the parts of this kind have been made of aluminum alloys since the early stage to take an advantage of the reduction in weight. Recently, parts such as a cylinder head and a crankshaft have been also made lightweight.
Meanwhile, it was believed difficult to form a cylinder liner with an aluminum alloy because higher high-temperature dimensional stability, higher abrasion resistance, greater strength, and greater rigidity are required due to the size, function, and operation of the cylinder liner. Thus, a metal matrix composite (MMC), i.e., a composite based on an aluminum alloy reinforced with metal and ceramic fibers or ceramic particles has been used to reduce the weight of the cylinder liner.
Conventionally, MMC cylinder liners such as those disclosed in the JP11-222638A, JP2007-508147A, JP2003-181620A, and JP06-170515A have been known. In addition, a method for producing an MMC cylinder liner such as one disclosed in Japanese Examined Patent Publication No. 03-003539 has been known. JP11-222638A describes an MMC cylinder liner based on a hypoeutectic Al—Si alloy wherein Si concentration is between 9.6 and 12. JP2003-181620A describes an MMC cylinder liner based on a hypoeutectic Al—Si alloy such as ADC12.
JP06-170515A describes a method for producing an MMC cylinder liner comprising the steps as illustrated in FIGS. 10A-D.
1) First, metal and ceramic fibers, which form a reinforcing material together, are hardened to form a porous tubular compact 50 that is made of the reinforcing material and that has a generally circular cross-section. As illustrated in FIG. 10A, the compact 50 is fitted over a generally cylindrical core 52 provided in a movable mold 51.2) As illustrated in FIG. 10B, the movable mold 51 is moved toward to a fixed mold 53. Then, a cavity 54 in a tubular form having a generally circular cross-section is formed around an outer periphery of the compact 50.3) As illustrated in FIG. 10C, a melted Al—Si alloy is supplied under the pressure from a gate 55 provided in the fixed mold 53 to the cavity 54 to cause the compact 50 to be impregnated with the melted Al—Si alloy.4) After the hardened Al—Si alloy was removed, an MMC cylinder liner as illustrated in FIG. 10D is produced. The cylinder liner includes an inner tubular portion 56 formed of a metal matrix composite and an outer tubular portion 57 formed of the Al—Si alloy.
The MMC cylinder liner as produced above is fused metallurgically with a body of a cylinder block during casting of the block.
For such MMC cylinder liners, adhesiveness to the body of the cylinder block during casting is required. As described in JP11-222638A and JP2003-181620A, when a low-hypoeutectic alloy having a low melting point is used as an Al—Si alloy forming an MMC cylinder liner, adhesiveness of the cylinder liner to the cylinder block is ensured. However, in that case, mechanical characteristics required for the inner peripheral surface of the cylinder liner that serves as a sliding surface of a piston may not be achieved. This may cause decrease in durability or add the necessity of further reinforcement with an increased amount of the reinforcing material.